The Long QT Syndrome (LQTS), an hereditary disorder involving about 10,000 persons in the U.S., is associated with delayed repolarization (yields QTc interval on ECG), paroxysmal ventricular arrhythmias, syncope, and sudden death. Recently, four genetic forms of LQTS have been identified including LQT3, a sodium-channel gene mutation (SCN5A, deltaKPQ deletion) with impairment of sodium-channel inactivation. The primary aims of this study of LQT3 patients are to determine: 1) if a low dose of the oral sodium-channel blocking drug mexiletine significantly shortens QTc by greater than or equal to 40 msec; and 2) if chronic administration of mexiletine is associated with sustained QTc shortening and a reduction in arrhythmic cardiac events. The study consists of two related parts: 1) a short-term (7-week), randomized, double-blind, placebo-controlled, crossover, dose-ranging study with oral mexiletine to determine if a low dose of mexiletine (1/4 or 1/2 of the standard dose) is as effective as a standard dose of mexiletine in significantly shortening the QTc interval; and 2) a long-term (38-month), randomized, double-blind, placebo-controlled, crossover, safety and efficacy study using the lowest effective mexiletine dose identified in part 1 to determine if chronic administration of this dose of mexiletine is associated with sustained shortening of the QTc interval and absence or a reduction of arrhythmic cardiac events when compared to placebo therapy. Forty LQT3 patients with genetically defined deltaKPQ deletion of the mutant sodium channel will be enrolled in the two parts of the i study. The study will be conducted in three clinical centers (Rochester, NY; Sioux City, IA; and Pavia, Italy) with the coordination, data management, and analysis center in Rochester. Clinical follow-up of the patients will include periodic digital 12-lead and high-resolution ECGs for quantitative QTc measurements. To maximize the power to detect significant differences in the primary and secondary end points, the crossover design will allow each patient to serve as his/her own control in the analysis. The trial should provide new insight into molecular-based, antiarrhythmic therapy for an inherited channelopathy. The significance of this work relates to the future use of molecular therapeutics to treat ion-channel disorders associated with congenital and acquired cardiac repolarization disorders.